Cartridge type filters and screw-on filter units are known. A cartridge filter is generally constructed to be inserted inside a filter housing and has no integral housing of its own. A screw-on filter generally has an external housing which surrounds the filter and which routes the filtered fluid to and from the filter. Cartridge filters and the filters found within the outside housing of a screw-on filter unit are typically constructed of a cylindrical shape with the filter media disposed radially along an outside surface of the filter. The filter media is typically pleated in some manner along the longitudinal axis of the filter to increase the overall filtering surface.
The filter media is typically supported by a filter media support structure which is also typically pleated along a longitudinal axis. The filter media support structure is, in turn, supported by a pair of end caps at least one of which may engage a filter housing and direct fluid flow along an outside surface of the filter through the filter media and to a filter return path lying along a center axis of the filter.
The filter media used within filters is often a woven material that may consist of a single layer of filtering material or several layers. The material used depends on the aggressiveness of the fluid to be filtered, the viscosity of the fluid, the abrasive grit or other contaminants to be removed from the filtered fluid and the flow the filtered fluid.
Where the filtered fluid is viscous or when contaminants begin to accumulate in the filter media partially blocking flow, a pressure differential develops across the filter. The pressure differential can cause the filter support structure to compress axially, causing the spaces between adjacent pleats of the pleated filter structure to close up (nest) further blocking flow of the filtered fluid.
Reduced flow of filtered fluids can lead to damage where the filtered fluid is oil and the oil serves to lubricate an internal combustion engine. Reduced flow can also lead to damage in hydraulic systems where the filtered fluid supplies a hydraulic pump and where cavitation caused by a partial blockage of fluid flow can lead to erosion of the structure of the pump. Because of the importance of fluid filtering, a need exists for a mechanism to reduce pressure induced blockage of filters caused by pressure differentials.